In the field of medical diagnostics, real-time observation of body processes detected by ultrasonic scans is conventionally done on television monitors. The echoes received by the ultrasonic probe are translated by a scan converter into luminance signals appropriately positioned on the monitor screen to form an image of the patient's internal organs.
In order to synchronize the ultrasonic scan with the standard television format, it is desirable to provide an array of ultrasonic transducers, successive portions of which are pulsed during the horizontal retrace intervals of the television picture. Because of the fact that some vital organs need to be observed through the rib cage, it is often necessary to produce a sector scan, i.e. a fan-shaped group of ultrasonic beams which is centered between two ribs near the skin surface and subtends an angle of about 90.degree. .
Several methods of achieving this result have been proposed. For example, a linear array of transducers may be placed against the patient's skin, and the beam may be steered in an arc by changing the phase of the elements of the array. Because of the accuracy required for this type of operation phased arrays are very expensive.
In U.S. Pat. No. 4,281,550 Erikson, it has been proposed to accomplish this result by means of a stationary probe containing a curved array of individually switchable transducers disposed along an arc of a circle whose center roughly coincides with an acoustically transparent window at which the probe is applied to the patient's skin.
Accuracy requirements for this type of probe are about an order of magnitude lower, because the scanning is done by mere translation of the beam along the array, while the steering is done by the geometry of the array.
The disadvantage of the curved-array probe is that, inasmuch as the array cannot be directly applied to the patient's skin, an acoustic fluid must be interposed within the probe between the array and the window. The easiest way of minimizing refraction, distortion and artifacts at the fluid-window-skin interface is to use a fluid (e.g. water) which matches as closely as possible the acoustic velocity of the skin. Unfortunately, the need to prevent interface reflection artifacts from appearing in the image dictates that the round-trip propagation time between the array and the window be greater than the round-trip propagation time of the beam between the window and the farthest point in the patient's body to be examined. This causes the probe to be large, heavy, and clumsy to handle. In addition, the beam-spread loss characteristics of water dictate the use of a large active array area requiring large numbers of transducer elements and complex electronics.
It was proposed in U.S. Pat. No. 4,242,912 to Burckhardt that a fluid with a relatively slow sound propagation velocity be used as the coupling fluid in an ultrasonic probe. U.S. Pat. No. 4,391,281 to Green subsequently proposed, in connection with a probe containing a mechanically oscillating transducer, that the probe fluid be a fluorocarbon, specifically Fluorinert FC 75 manufactured by 3M Company. However, these teachings were not directly applicable to curved-array probes because the use of a slow fluid in a curved-array probe causes reflection, refraction, and distortion problems which long defied practical solution.
Specifically, it was found that because of the refraction of the beam at the fluid-window-skin interface (which allows a slow-fluid curved-array probe to be much lighter, smaller, cheaper, and less complex than a water-filled one), the stiffness of the window became critical. For example, with the highly desirable Fluorinert FC 72 as the probe fluid, a movement of as little as 0.15 mm in the window surface causes the focus of the beam to vary between 8 cm and infinity. The only way to prevent such movement when the probe was applied to a patient's skin was to provide a thermally stable window of high structural integrity to maintain its shape. Such a window, however, would have to be made of a thick, hard material which, it was generally believed, would inherently have a very high acoustic velocity. Not only would such a material prevent scanning the beam through 90.degree. in the patient's body (because the slow fluid-hard window interface would, due to Snell's law, cause total reflection at even a low angle of incidence from the vertical), but it would also cause unacceptable multiple reflections and scattering from the probe walls sufficient to degrade the image beyond commercial practicality. For this reason, slow fluid was never in fact used in a commercial curved-array probe.